If they end up examining someone's personal e-mail by mistake, they could find themselves in court

The TUC's Sarah Veale

"The worst thing for companies is that it can affect their whole reputation in the marketplace," he says.

Providing software enabling employers to monitor internet use has been a lucrative business for Content Technologies.

It has already sold products to 6,000 customers worldwide that check the text of e-mails for breaches of confidentiality and company policy.

In the last few days, the security software giant Baltimore Technologies announced that it was buying Content Technologies for around £700m.

Union concern

But there is some concern that measures to monitor staff use of e-mail and the internet could go too far.

The unions say they fully support any measures to stamp out pornography in the workplace. But the TUC's Employment Rights officer, Sarah Veale, says staff must be made aware that surveillance is going on.

"Companies must also be aware that when the Human Rights Act comes into force next month, that gives employees the right to privacy in the workplace," she says.

"If they end up examining someone's personal e-mail by mistake, they could find themselves in court."

The UK Government is about to introduce new laws governing how far companies can go in intercepting e-mails as part of the Regulation of Investigatory Powers Act.

Employers fear that making the rules too tight will make it impossible for them to carry out their business securely.

The unions say staff must be given protection against firms using "Big Brother" tactics. The internet, and e-mail in particular, has become an indispensable tool for many businesses.

But working out ways of making sure the new technology is not misused is proving a major headache.